In subterranean well operations, it is necessary from time to time to remove a section of subsurface well conduit such as a tubing string or a well casing. Accordingly, several types of tubing and/or casing cutting and milling tools and procedures have been developed for use with conventional rotary drilling rigs. The cost and time consumed in using a conventional rotary drilling rig is considerable and there has been a trend towards the use of coiled tubing units for various well operations heretofore conducted with conventional drilling rigs.
Coiled tubing units are known in the art, but not widely used in the field yet. Coiled tubing units are nevertheless available on a commercial basis. Inventions such as that disclosed herein will render coiled tubing units more readily useful in the field by reducing both the cost and time expenditure, as compared to a conventional drilling rig, for a given operation.
Heretofore, tools and procedures have been developed for use with conventional drilling rigs for removing a section of a well conduit, whether it is tubing or casing, but these tools and procedures cannot be transferred unchanged to a coiled tubing unit and employed successfully in the same manner as employed in the conventional drilling rig. The use of conventional drilling rig tools and procedures in a coiled tubing context has several shortcomings. For example, control over the axial downward pressure on the tool or tools employed downhole is difficult to maintain because of the flexibility of the coiled tubing string. Accordingly, the cutting or milling tool may wear prematurely or unduly cut into other downhole tools such as whipstocks. The tools may also deflect the tubing being cut resulting in failure of the tools themselves and/or jamming of the tools in the tubing thereby causing an expensive fishing job or even abandonment of the well.
Further, conventional drilling rig cutting and milling tools are not adapted to be inserted into a casing string through a smaller diameter tubing string contained in that casing string. These types of tools require removal of the tubing string in its entirety from the casing and wellbore before the cutting and milling tools can be inserted into the casing and operated to form a window in that casing.
Also, conventional drilling rig cutting and milling tools are difficult to operate on a tubing string since, in many instances, the tubing string may be forced off center with respect to the longitudinal axis of the larger diameter casing string in which the tubing string to be cut and milled is located.
Finally, conventional rotary drilling rigs often put a very large amount of weight on the conduit cutting and milling tools in order to make up for the relatively slow rotational speed for a rotary rig, but this weight has a disadvantage of sometimes rotating and therefore disorienting the whipstock with the result that the window is not formed at the desired location in the well conduit. With a coiled tubing unit, the cutting and milling tools can be routinely rotated at much higher speeds than with a conventional rotary rig thereby eliminating the need for putting large amounts of weight on those tools in operation as is normally done with a conventional rotary rig.